Reading Atoms in Reactions

By the end of this lesson, you will be able to use the periodic table to figure out which atoms take part in a chemical reaction.

The periodic table is an organized chart of all the known elements, and each element has its own box with a one- or two-letter symbol, like O for oxygen, Fe for iron, and Na for sodium. Notice that when a symbol has two letters, the first one is capitalized and the second one is lowercase. That rule matters, because Co means the element cobalt, while CO means one carbon atom joined to one oxygen atom.

Think about a bike left outside during a rainy Texas week. The shiny metal turns orange and flaky. That change is a chemical reaction, which means the atoms in the starting materials rearrange to form new substances with new properties. Scientists write it like this: iron plus oxygen makes iron oxide, or Fe + O2 becomes Fe2O3. To identify the atoms involved, you look up each symbol on the periodic table. Fe sends you to iron, and O sends you to oxygen. So only two kinds of atoms are in this whole reaction.

The small numbers written low and to the right are called subscripts, and they tell you how many atoms of that element are in one unit of the substance. In Fe2O3, the 2 means two iron atoms and the 3 means three oxygen atoms. Counting these numbers helps you prove that matter is conserved, because the same atoms that start the reaction are still there at the end, just rearranged into a new arrangement.

So your job is simple: find each symbol, match it to an element on the periodic table, and count the atoms.

Try it: in the reaction that happens when baking soda reacts and releases carbon dioxide, CO2, which two elements does the periodic table tell you are involved, and how many atoms of each are in one molecule?